1,255 research outputs found
A low-voltage retarding-field Mott polarimeter for photocathode characterization
Nuclear physics experiments at Thomas Jefferson National Accelerator
Facility's CEBAF rely on high polarization electron beams. We describe a
recently commissioned system for prequalifying and studying photocathodes for
CEBAF with a load-locked, low-voltage polarized electron source coupled to a
compact retarding-field Mott polarimeter. The polarimeter uses simplified
electrode structures and operates from 5 to 30 kV. The effective Sherman
function for this device has been calibrated by comparison with the CEBAF 5 MeV
Mott polarimeter. For elastic scattering from a thick gold target at 20 keV,
the effective Sherman function is 0.201(5). Its maximum efficiency at 20 keV,
defined as the detected count rate divided by the incident particle current, is
5.4(2) x 10-4, yielding a figure-of-merit, or analyzing power squared times
efficiency, of 1.0(1) x 10-5. The operating parameters of this new polarimeter
design are compared to previously published data for other compact Mott
polarimeters of the retarding-field type.Comment: 9 figure
Measuring the Reduced Shear
Neglecting the second order corrections in weak lensing measurements can lead
to a few percent uncertainties on cosmic shears, and becomes more important for
cluster lensing mass reconstructions. Existing methods which claim to measure
the reduced shears are not necessarily accurate to the second order when a
point spread function (PSF) is present. We show that the method of Zhang (2008)
exactly measures the reduced shears at the second order level in the presence
of PSF. A simple theorem is provided for further confirming our calculation,
and for judging the accuracy of any shear measurement method at the second
order based on its properties at the first order. The method of Zhang (2008) is
well defined mathematically. It does not require assumptions on the
morphologies of galaxies and the PSF. To reach a sub-percent level accuracy,
the CCD pixel size is required to be not larger than 1/3 of the Full Width at
Half Maximum (FWHM) of the PSF. Using a large ensemble (> 10^7) of mock
galaxies of unrestricted morphologies, we find that contaminations to the shear
signals from the noise of background photons can be removed in a well defined
way because they are not correlated with the source shapes. The residual shear
measurement errors due to background noise are consistent with zero at the
sub-percent level even when the amplitude of such noise reaches about 1/10 of
the source flux within the half-light radius of the source. This limit can in
principle be extended further with a larger galaxy ensemble in our simulations.
On the other hand, the source Poisson noise remains to be a cause of systematic
errors. For a sub-percent level accuracy, our method requires the amplitude of
the source Poisson noise to be less than 1/80 ~ 1/100 of the source flux within
the half-light radius of the source, corresponding to collecting roughly 10^4
source photons.Comment: 18 pages, 3 figures, 4 tables, minor changes from the previous
versio
Chemical telemetry of OH observed to measure interstellar magnetic fields
We present models for the chemistry in gas moving towards the ionization
front of an HII region. When it is far from the ionization front, the gas is
highly depleted of elements more massive than helium. However, as it approaches
the ionization front, ices are destroyed and species formed on the grain
surfaces are injected into the gas phase. Photodissociation removes gas phase
molecular species as the gas flows towards the ionization front. We identify
models for which the OH column densities are comparable to those measured in
observations undertaken to study the magnetic fields in star forming regions
and give results for the column densities of other species that should be
abundant if the observed OH arises through a combination of the liberation of
H2O from surfaces and photodissociation. They include CH3OH, H2CO, and H2S.
Observations of these other species may help establish the nature of the OH
spatial distribution in the clouds, which is important for the interpretation
of the magnetic field results.Comment: 11 pages, 2 figures, accepted by Astrophysics and Space Scienc
Topological mirror symmetry with fluxes
Motivated by SU(3) structure compactifications, we show explicitly how to
construct half--flat topological mirrors to Calabi--Yau manifolds with NS
fluxes. Units of flux are exchanged with torsion factors in the cohomology of
the mirror; this is the topological complement of previous
differential--geometric mirror rules. The construction modifies explicit SYZ
fibrations for compact Calabi--Yaus. The results are of independent interest
for SU(3) compactifications. For example one can exhibit explicitly which
massive forms should be used for Kaluza--Klein reduction, proving previous
conjectures. Formality shows that these forms carry no topological information;
this is also confirmed by infrared limits and old classification theorems.Comment: 35 pages, 5 figure
Neutron scattering and molecular correlations in a supercooled liquid
We show that the intermediate scattering function for neutron
scattering (ns) can be expanded naturely with respect to a set of molecular
correlation functions that give a complete description of the translational and
orientational two-point correlations in the liquid. The general properties of
this expansion are discussed with special focus on the -dependence and hints
for a (partial) determination of the molecular correlation functions from
neutron scattering results are given. The resulting representation of the
static structure factor is studied in detail for a model system using
data from a molecular dynamics simulation of a supercooled liquid of rigid
diatomic molecules. The comparison between the exact result for and
different approximations that result from a truncation of the series
representation demonstrates its good convergence for the given model system. On
the other hand it shows explicitly that the coupling between translational
(TDOF) and orientational degrees of freedom (ODOF) of each molecule and
rotational motion of different molecules can not be neglected in the
supercooled regime.Further we report the existence of a prepeak in the
ns-static structure factor of the examined fragile glassformer, demonstrating
that prepeaks can occur even in the most simple molecular liquids. Besides
examining the dependence of the prepeak on the scattering length and the
temperature we use the expansion of into molecular correlation
functions to point out intermediate range orientational order as its principle
origin.Comment: 13 pages, 7 figure
Synchronization in a System of Globally Coupled Oscillators with Time Delay
We study the synchronization phenomena in a system of globally coupled
oscillators with time delay in the coupling. The self-consistency equations for
the order parameter are derived, which depend explicitly on the amount of
delay. Analysis of these equations reveals that the system in general exhibits
discontinuous transitions in addition to the usual continuous transition,
between the incoherent state and a multitude of coherent states with different
synchronization frequencies. In particular, the phase diagram is obtained on
the plane of the coupling strength and the delay time, and ubiquity of
multistability as well as suppression of the synchronization frequency is
manifested. Numerical simulations are also performed to give consistent
results
Synchronization and resonance in a driven system of coupled oscillators
We study the noise effects in a driven system of globally coupled
oscillators, with particular attention to the interplay between driving and
noise. The self-consistency equation for the order parameter, which measures
the collective synchronization of the system, is derived; it is found that the
total order parameter decreases monotonically with noise, indicating overall
suppression of synchronization. Still, for large coupling strengths, there
exists an optimal noise level at which the periodic (ac) component of the order
parameter reaches its maximum. The response of the phase velocity is also
examined and found to display resonance behavior.Comment: 17 pages, 3 figure
Phase synchronization and noise-induced resonance in systems of coupled oscillators
We study synchronization and noise-induced resonance phenomena in systems of
globally coupled oscillators, each possessing finite inertia. The behavior of
the order parameter, which measures collective synchronization of the system,
is investigated as the noise level and the coupling strength are varied, and
hysteretic behavior is manifested. The power spectrum of the phase velocity is
also examined and the quality factor as well as the response function is
obtained to reveal noise-induced resonance behavior.Comment: to be published in Phys. Rev.
The PL calibration for Milky Way Cepheids and its implications for the distance scale
The rationale behind recent calibrations of the Cepheid PL relation using the
Wesenheit formulation is reviewed and reanalyzed, and it is shown that recent
conclusions regarding a possible change in slope of the PL relation for
short-period and long-period Cepheids are tied to a pathological distribution
of HST calibrators within the instability strip. A recalibration of the
period-luminosity relation is obtained using Galactic Cepheids in open clusters
and groups, the resulting relationship, described by log L/L_sun =
2.415(+-0.035) + 1.148(+-0.044)log P, exhibiting only the moderate scatter
expected from color spread within the instability strip. The relationship is
confirmed by Cepheids with HST parallaxes, although without the need for
Lutz-Kelker corrections, and in general by Cepheids with revised Hipparcos
parallaxes, albeit with concerns about the cited precisions of the latter. A
Wesenheit formulation of Wv = -2.259(+-0.083) - 4.185(+-0.103)log P for
Galactic Cepheids is tested successfully using Cepheids in the inner regions of
the galaxy NGC 4258, confirming the independent geometrical distance
established for the galaxy from OH masers. Differences between the extinction
properties of interstellar and extragalactic dust may yet play an important
role in the further calibration of the Cepheid PL relation and its application
to the extragalactic distance scale.Comment: Accepted for Publication (Astrophysics & Space Science
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